In many circumstances an individual may require cooling or warming relative to an ambient environment in order to remain comfortable and, in the case of some extreme environments, in order to maintain performance and health.
In some situations the individual may be located inside a structure or the like; in this case, it is generally most efficient to cool or warm the interior of the structure, e.g., with air conditioning or heaters. This approach is generally convenient in that it allows more than one individual to be cooled or heated at the same time, and it allows the individual to remain in relative comfort regardless of their precise location within the structure.
In other situations the individual may be located in an exterior environment; in this case, it is generally not practical to cool or warm the ambient environment around the individual, particularly if the individual is moving about to a significant degree. However, where it is very hot (e.g., the southwestern border of the continental United States during the summer) or very cold (e.g., the northern border of the continental United States during the winter), and where the individual must be outdoors for prolonged periods of time (e.g., Immigration and Naturalization Service officers, other law enforcement officers, military personnel, sportsmen, etc.), it can be imperative that the individual be provided with a system to provide them with effective cooling or heating while they are outdoors.
It can also be important to provide a personal cooling or warming system for an individual when that individual must be enclosed within an impermeable protective suit, e.g., a chemical weapons protective suit, a bio-weapons protective suit, a radioactivity protective suit, etc.